Many of today's industrial as well as personal electronic equipment rely on microcontrollers or digital signal processors (“DSP”) to process real world, analog signals. For example, sensors can be used to convert a natural parameter such as temperature or pressure into a voltage or current. Analog-to-digital converter (“ADC”) can then transform the signals into a digital format. Typically, an ADC converts an input analog voltage to discrete digital values and a number of ADC input formats exist including single-ended and differential inputs.
Single-ended signaling is a widely used technique and is the simplest method for transmitting electrical signals over wires. In single-ended applications, one wire carries a varying voltage that represents the signal, while another wire is connected to a reference voltage, usually ground. The primary advantage of using single-ended inputs is that a minimal number of wires are needed to transmit multiple signals. While single-ended inputs may be ideal where the signal source and ADC are close to each other, such inputs are more susceptible to coupled-noise and DC offsets. Differential signaling, on the other hand, is a method for transmitting information over pairs of wires. In differential applications, two wires are routed in parallel, and sometimes twisted together so that both wires receive the same interference. In this configuration, one wire carries the signal while the second wire carries the inverse of the signal. Advantages of differential signaling include noise reduction by rejecting common-mode interference and greater dynamic range.
Depending on the nature of an application, single-ended signaling may be more appropriate than differential signaling and vice versa. For example, an application sensitive to noise may necessitate the use of differential signals. Some applications, however, lack signals preconditioned for differential operation and a single-ended-to-differential conversion stage is required.
To maintain a low pulse-width distortion for a low deterministic jitter, existing single-ended-to differential conversion methods employ a DC-cancellation loop to set up a decision threshold at 50% of the single-ended input data transition. The DC-cancellation loop introduces a time constant for stabilizing the decision threshold. While systems including DC-cancellation loops can be used for continuous-mode and balanced data transmission, they are inadequate for some applications. In a burst-mode application, for example, valid differential data must be established within very limited unit interval counts (e.g., the symbol duration time) and the decision threshold has to be maintained over an extended sequence of data stream. Moreover, for un-balanced data transmission such as 4B/5B coding, the conventional DC-cancellation technique would introduce a pulse-width distortion at the differential output, which would then drain the system jitter margin. Accordingly, there is a need for converting single-ended signals to differential signals without using a DC-cancellation loop.